1. Field of the Invention
The present invention relates to an image-forming member for electrophotography which is sensitive to electromagnetic wave such as light including for example ultraviolet ray, visible ray, infrared ray, x ray and gamma ray.
2. Description of the Prior Art
Photoconductive materials for constituting a photoconductive layer in an image-forming member for electrophotography are required to exhibit various properties, for example high sensitivity, high resistance, spectral characteristics as close to luminosity as possible, high speed of light response, large coefficient of light absorption in the range of visible light and excellent stability to external influence such as light, heat and the like. In addition, they are required to be non-harmful or hardly harmful to man.
Particularly, in case of an electrophotographic image-forming member incorporated into an electrophotographic apparatus used as office supplies, a problem of harmfulness during use of the apparatus is very important and serious. However, it can be hardly asserted positively that materials of the prior art, for example inorganic photoconductive materials such as Se, CdS, ZnO and the like, and organic photoconductive materials (PVC.sub.z), trinitrofluorenone (TNF) and the like always satisfy all of the foregoing requirements over a certain level.
For example, an electrophotographic image-forming member provided with an Se-type photoconductive layer to which Te or As is incorporated possesses improved spectral sensitivity range. However, it is inadvantageous that since its light fatigue become larger, when copying operation is continuously repeated with the same, one original, the image density of the copied images is decreased, and the background of the images is stained, that is, fogging phenomenon takes place in the white ground. Further, when the copying operation is successively reopened by using a new original, undesired images are obtained in which images of the last original inadvantageously appear as residual images, that is, ghost phenomenon takes place.
Inorganic photoconductive materials such as CdS, ZnO and the like are used for so-called binder type photoconductive layer which is formed by processing the materials into granular form and dispersing them into an organic polymerizable binder of electrically insulating property. However, the binder type photoconductive layer is essentially composed of two components, i.e. photoconductive material and resin binder and required to be a system in which the photoconductive material particles must be uniformly dispersed into the binder. As a result, such photoconductive layer includes many parameters for determining electric, photoconductive, physical and chemical properties thereof. Therefore, if such many parameters are not carefully controlled a photoconductive layer having the desired properties cannot be obtained with good reproducibility. It is further inevitable that the yield is decreased so that such photoconductive layer is lacking in the mass-producibility.
The photoconductive layer of binder type is porous as a whole due to a special structure of dispersion system so that it depends greatly upon humidity. When it is used in the atmosphere of a high humidity, its electric property is deteriorated. As a result, there are not a few cases in which copied images of high quality cannot be obtained.
Further, owing to the porosity of the binder type photoconductive layer, developer is allowed to enter into the layer, which results in deteriorating release property and cleaning property and ultimately leads to impossible use. In particular, when the used developer is a liquid developer, it penetrates into the photoconductive layer along with the carrier solvent by capillary action so that the above disadvantages are enhanced.
Electrophotographic image-forming members using organic photoconductive materials such as poly-N-vinylcarbazole, trinitrofluorenone and the like have such drawbacks that they are lacking in moisture resistance, corona ion resistance and cleaning property and have only low photosensitivity and narrow spectral sensitivity range to the visible light region with the sensitivity being partial to a shorter wave length region. Therefore, such members are used only in the extremely restricted field.
In view of the foregoing, it is desired to develop a third material for providing a photoconductive layer free from the above-mentioned drawbacks.
Such a material is, for example amorphous silicon (hereinafter called "a-Si") which is recently considered to be promising. At the beginning of developing an a-Si layer, its structure varies depending upon the producing methods and conditions so that its electric and optical properties also vary and the reproducibility is questionable. However, in 1976 success of producing p-n junction in a-Si, which has been considered impossible, was reported (Applied Physics Letters, Vol. 28, No. 2, pp. 105-107, Jan. 15, 1976). Since then, the a-Si draws attentions of scientists and is studied and developed for application mainly to solar cells.
However, in practice, such an a-Si developed for solar cell cannot be directly used as a material for a photoconductive layer of an electrophotographic image-forming member from the viewpoints of its electric, optical and photoconductive properties. Solar cells take out solar energy in the form of electric current, and therefore the a-Si film must have a relatively low resistance for the purpose of obtaining efficiently the electric current with a good SN ratio, i.e. photo-current (ip) / dark current (id), but if the resistance is too low, the photosensitivity is deteriorated and the SN ratio is degraded. Therefore, the resistance should be 10.sup.5 -10.sup.8 ohm.multidot.cm.
However, such a degree of resistance (dark resistance) is so low for a photoconductive layer of an electrophotographic image-forming member that such an a-Si film cannot be used for the photoconductive layer.
Further, reports concerning a-Si films disclose that when the dark resistance is increased, the photosensitivity is lowered. For example, an a-Si film having a dark resistance of about 10.sup.10 ohm.multidot.cm shows a lowered photoconductive gain, i.e. photocurrent per incident photon. Therefore, the conventional a-Si films cannot be used for a photoconductive layer even from this point of view. In addition, an electrophotographic image-forming member of two-layer structure including a photoconductive layer of the conventional a-Si and a substrate exhibits high speed of dark decay, in other words, poor charge retentivity. Therefore, such an image-forming member cannot provide satisfactory images or perform any image formation at a process speed for the electrophotographic process as known at present.
The conventional a-Si has additionally many drawbacks to be resolved. For example, the a-Si cannot be given a uniform photosensitivity to the whole region of the visible light, particularly with the sensitivity being lowered at the side of shorter wave length in the vicinity of 400 nm. In order to produce an a-Si layer having desired properties over a large area, the producing conditions must be carefully controlled. The layer growth ratio of a-Si is remarkably low, for example as low as about 1/100 of that of Se and the like, which requires careful control of the layer-forming conditions for a long period of time in case of obtaining a layer having a sufficient thickness for a photoconductive layer of electrophotographic image-forming member. In some cases, it is necessary to retain the layer-forming conditions constantly.
The present invention has been accomplished in the light of the foregoing. The present inventors have continued researches and investigations with great zeal concerning many photoconductive materials including a-Si from a viewpoint that a-Si is applied to a photoconductive layer of an electrophotographic image forming member without damaging the advantages of the a-Si. As a result, they have succeeded in designing and manufacturing electrophotographic imageforming members which are able to eliminate all problems as mentioned above.